Sexual reproduction — fusion of gamete nuclei
Sexual reproduction is the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other and from both parents. Both halves are needed for full credit. Each gamete carries its own combination of genetic information, so every zygote gets a different mixture. Contrast asexual reproduction (topic 16.1): one parent, no fusion, genetically identical offspring.
Fertilisation, haploid and diploid
Fertilisation is the fusion of the nuclei of gametes — the nuclear fusion step itself, not an earlier stage such as pollen reaching a stigma. (Extended) Every gamete nucleus is haploid: one set of chromosomes. When two fuse, the zygote nucleus is diploid: two sets, one from each parent. Fertilisation restores the chromosome number that meiosis halved when the gametes formed, and every body cell grown from the zygote by mitosis stays diploid.
(Extended) Meiosis is where the variation comes from
Meiosis is the nuclear division that forms gametes, and the origin of variation in sexual reproduction. It produces four genetically different haploid gametes, each with its own allele combination. Mitosis produces two genetically identical diploid cells for growth and repair, and forms no gametes. Two parents each supply a gamete, so fertilisation combines two unique sets — which is why sexual reproduction supplies the variation natural selection acts on.
Drawn from real examiner reports.
Fertilisation ≠ pollination
Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation is the fusion of the nuclei of gametes, happening later — once the pollen tube has delivered the pollen nucleus into the ovule. Pollination comes first but is a separate event: "the pollen grain reaches the stigma" describes pollination and earns nothing as a definition.
Fertilisation vs. pollination confused: some candidates stated fertilisation is when pollen lands on the stigma — that is pollination; fertilisation is when the pollen nucleus fuses with the nucleus in the ovule (s23 Paper 13 Q28).
Meiosis makes variation, not mitosis
Mitosis produces two genetically identical diploid cells — growth and repair, never gametes. Meiosis produces four genetically different haploid gametes — the only source of variation. "Mitosis produces the variation" is an examiner-reported error. Safe sentence: meiosis makes different gametes, and fertilisation carries that variation into the zygote.
Meiosis vs. mitosis features known but names swapped (w22 Paper 31 Q2c); mitosis produces genetically identical cells, meiosis produces genetic variation — candidates stated mitosis increases genetic variation (w22 Paper 12 Q35).
Spell mitosis and meiosis apart
Examiners have withheld credit for phonetic spellings — "meitosis" for mitosis, "meosis" for meiosis — because the marker cannot tell which process is meant. The two words differ by a syllable but name opposite outcomes (identical cells versus varied gametes), so an ambiguous spelling costs the mark even when the biology is right.
Meiosis vs. mitosis spelling: "meitosis" for mitosis; "meosis" for meiosis — phonetic spellings occasionally too ambiguous (s22 Paper 41 Q6a).
Gametes haploid, zygote diploid
The direction gets reversed surprisingly often. A gamete nucleus — sperm, egg cell, pollen, ovule — is haploid: one set of chromosomes. The zygote nucleus formed at fertilisation is diploid: two sets, one per parent. "The gamete is diploid and splits to make a haploid zygote" inverts both halves and scores zero. This detail is Supplement (Extended) only.
Ovulation is not meiosis
Ovulation is the release of an egg cell from the ovary — an event in the cycle, not a cell division. Meiosis is the nuclear division that produced that egg cell in the first place. Candidates asked to name the division that forms gametes have answered "ovulation", which describes a later and unrelated step, so it gains no credit. Name the division, never the release.
Ovulation (release of egg cell from ovary) confused with meiosis (the cell division that produces egg cells) (s22 Paper 31 Q4a ii).
"Two parents" is not a definition
Vague answers to "describe what is meant by sexual reproduction" — "two parents make a baby", "reproduction using a male and a female" — score no marks. The mark scheme wants the fusion of the nuclei of two gametes to form a zygote, plus genetically different offspring. Say what fuses, then say what the offspring are like.
Answer both named contexts
Extended statement 16.2.4 asks for advantages and disadvantages in two contexts: a population in the wild, and crop production. Give one advantage and one disadvantage for each, labelled by context. An answer covering only one context cannot reach full marks.
The four points worth banking
Wild population — advantage: varied offspring mean some survive a change; disadvantage: finding a mate or pollinator costs time, so fewer offspring. Crop production — advantage: variation lets breeders select improved varieties; disadvantage: offspring are unpredictable.
Reuse the variation chain
One causal chain answers most of these: meiosis makes varied gametes → some offspring inherit alleles suited to a changed environment → those individuals survive and reproduce → the population persists or is selectively bred. The marks sit in the links, not the word "variation".
Match the command word
"State" wants the term only. "Describe what is meant by" wants the full two-element definition. "Discuss" wants an advantage and a disadvantage, each with a reason, in both named contexts. "Explain" wants a cause-and-effect link, not a fact restated in different words.
Sexual reproduction: a process involving the fusion of the nuclei of two gametes to form a zygote, and the production of offspring that are genetically different from each other.
Fertilisation: the fusion of the nuclei of gametes. (In animals, this is the sperm nucleus fusing with the egg cell nucleus; in flowering plants, the pollen nucleus fusing with a nucleus in the ovule.)
Gamete: a sex cell (e.g. sperm, egg cell, pollen nucleus, ovule) that fuses with another gamete at fertilisation.
Zygote: the single diploid cell formed immediately after fertilisation, from which the new organism develops.
(Extended) Haploid: a nucleus containing one complete set of chromosomes — this is the state of a gamete nucleus.
(Extended) Diploid: a nucleus containing two complete sets of chromosomes (one from each parent) — this is the state of a zygote nucleus, and of ordinary body cells.
Define sexual reproduction.
(a) Describe what is meant by sexual reproduction.
(b) Describe what is meant by fertilisation.
